Various methods of liquifying gaseous carbon dioxide are well known. Typically, the liquification process comprises compressing the gaseous carbon dioxide to a pressure above atmospheric pressure and then removing the latent heat of vaporization to condense the compressed gas. In this way, although the sublimation temperature of solid carbon dioxide is approximately -109.9.degree. F. at STP, the compressed gaseous carbon dioxide can be condensed at much higher temperatures.
The theoretical range of pressures over which gaseous carbon dioxide can be condensed to a liquid is approximately 60.45 to 1057.4 psig. However, most commercial processes operate in the range of approximately 225 to 300 psig. In this range, the temperature at which gaseous carbon dioxide gas will condense is -14.degree. F. at 225.25 psig and -8.degree. F. at 251.96 psig. Therefore, it will be appreciated that liquification can be accomplished without the use of sophisticated refrigeration equipment to achieve very low temperatures. Rather, standard refrigeration systems using ammonia or fluorocarbon refrigerants, such as freon, can be used.
Liquified carbon dioxide has many applications, but is particularly useful in the beverage industry for carbonating beverages, such as beer and soft drinks. However, in order for the carbon dioxide to be of maximum usefulness, it must be as pure as possible, i.e. free of contaminating gas such as oxygen and to a lesser extent nitrogen. If the carbon dioxide contains a significant amount of oxygen, the beverage in which it is used will be subject to oxidation and spoilage.
In a typical liquification apparatus, gaseous carbon dioxide is passed through a tube which is surrounded by a refrigerant. The carbon dioxide condenses on the sides of the tube and collects in the bottom thereof. Since the tube is filled with gaseous carbon dioxide, there is a large surface area at the interface of the liquid carbon dioxide and the gaseous carbon dioxide. This condition permits contaminating gases in the gaseous carbon dioxide to combine with the liquid carbon dioxide, not by liquification, but by solution. As a result, the liquid carbon dioxide is contaminated with undesirable solubilized gaseous substances, such as oxygen and nitrogen.
Heretofore a system for economically providing substantially pure liquified carbon dioxide has not been known.